You could edit your theme to enable additional regions. For example Handlebar theme has several regions as pop up (modal) regions and also a 'top bar'.
The top bar is similar to what you want, but actually goes across the top of both the main column and the sidebar, but you could take that and amend it slightly, bringing it into the main content area rather than across the top of everything.
Another alternative might be to create your plugin as an activity/resource (mod) rather than a block. Then you could add that activity into any content section.